Manufacturing of products and packages often requires transforming a continuous flat web of material into individual products and packages. For example, soluble unit dose fabric and dish care pouches are formed from flat webs of water soluble film that are converted into three dimensional pouches by shaping and assembling layers of film. Similarly, diapers, sanitary napkins, wipes, bandages, and the like are formed by layering multiple flat webs of material upon one another and cutting the layered webs to form individual products comprised of multiple layers of material.
Webs of material can be cut in the cross direction by passing the web through the nip of a rotary press having a cutting knife mounted thereon and an anvil. As the web passes through the nip between the press and the anvil, the cutting knife strikes the web and cuts the web. To provide for a consistently complete cut of the web in the cross direction, the rotary press and anvil are set so that there is interference between the cutting knife and the anvil. That is, the rotary press and anvil are set so close to one another that cutting knife must slightly deform to permit the rotary press and the anvil to counter rotate with one another. For instance the knife may have a height of 40 mm and the peripheral surfaces of the rotary press and anvil are set such that they are only 39.9 mm apart. Thus, when the web of material is fed through the nip between the rotary press and the anvil, deformation or movement of 0.1 mm must be provided to permit the knife to pass through the nip between the surface of the rotary press and the anvil.
Ordinarily, most of the deformation is desirably provided by deformation of the knife as opposed to deformation or movement of the rotary press and or anvil. Movement of the axes of rotation of one or both of the rotary press and or anvil could result in a loss of control of movement of the web and fatigue of parts of expensive precision machine equipment. Typically anvils are formed of solid hardened material such as steel and little peripheral deformation occurs under typical cutting loads and stresses.
Since by design the knife accommodates most of the interference, the knife is loaded and unloaded each time the web is cut in the machine direction. Operators of converting lines loath having their lines shut down for maintenance. Accordingly, they try to design cutting systems on such converting lines to operate for extended periods with a minimal amount of down-time for maintenance. Ideally, operators would like to be able to make millions of cuts, and thus load and unload the knife millions of time, without shutting down the converting line. Loading and unloading of a knife mounted on a rotary press millions of time can result in fatigue of the knife, which ultimately can lead to failure of the knife. One technique for reducing fatigue in rotary cutting knives is the mount the cutting knife on the rotary press at an angle relative to the anvil so that the interference is accommodated by bending of the knife. A disadvantage of mounting a knife as such is that a variable speed rotary press operating at low speed may be needed to cut webs that are formed into three-dimensional shapes, such as for soluble unit dose fabric and dish care pouches.
With these limitations in mind, there is a continuing unaddressed need for a rotary press knife that has a long fatigue life. Surprisingly, the apparatus and process of the present invention improved the fatigue lifetime of the knife.